Heterodyne receivers convert received signals from the frequency at which they are received to an intermediate frequency (IF) by mixing the received signals with signals generated by a local oscillator. Such receivers are distinguishable from direct-conversion receivers, which convert the received signals directly to baseband. Superheterodyne receivers initially convert the received signals to a low-intermediate frequency (Low-IF) before converting the signals to baseband.
Heterodyne (and Superheterodyne) receivers are sensitive to various signal impairments that affect the quality of the received signals. Signal impairments may result from non-idealities in the RF front-ends of the receivers. For example, mismatched active and passive elements (e.g., quadrature mixers, filters, and/or analog-to-digital converters) in the I and Q (in-phase and quadrature) signal paths introduce I/Q mismatch impairments in transmitted and received signals. Channel effects may also impair signals.
Heterodyne receivers are particularly sensitive to imaging, which causes interference from unwanted frequencies. Specifically, a mixer that converts a desired frequency to IF simultaneously converts an undesired frequency to the same IF (e.g., fdesired=IF+fLO, where fLO is the local oscillator frequency; fundesired=IF−fLO). For example, a local oscillator signal having a frequency of 200 kHz may be used to convert a desired 700 kHz signal to a 500 kHz IF band signal. However, the local oscillator may also have the effect of converting an undesired 300 kHz signal to the 500 kHz IF band. As a result, any noise or unwanted data carried on the undesired frequency may interfere with the received data signal at the IF band. The undesired frequency is called the “image” of the desired frequency.